Process for producing elevated temperature dimensionally stable polyester articles with low gas permeability

Abstract

The invention provides a process of manufacturing a shaped polyester article having an oxygen permeability of from about 0.2 to about 4.9 cc-mil/100 in2-24 hr-atm at a temperature of 23 DEG C. and at 100% relative humidity inside and 60% relative humidity outside, and an enthalpy of recrystallization of from about 0 to about -2.1 cal/g, as well as a linear dimensional shrinkage between about 0% and 6% when heated from about -60 DEG C. to about 200 DEG C. The crystalline polyester articles are useful as containers for microwavable, ovenable and/or frozen foods, and have good gas barrier properties, and post-mold dimensional stability from about -60 DEG C. to about 200 DEG C.

What is claimed is: 
     
       1. A process for producing a shaped polyester article comprising the steps: (a) heating a composition consisting essentially of a substantially non-oriented crystallizable thermoplastic polyethylene terephthalate homopolymer to a completely or substantially completely amorphous state;   (b) maintaining the composition heated in step (a) at a temperature above the peak crystallization rate temperature of said crystallizable polyethylene terephthalate homopolymer, while simultaneously contacting said composition with a surface which imparts shape to said composition,   said surface being at or above the peak crystallization rate temperature from melt for said polyethylene terephthalate homopolymer;   (c) maintaining the contact between said composition and said surface, at the surface temperature set forth in step (b), for about 20 seconds to about 5 minutes to provide a crystalline composition having an enthalpy of recrystallization of about 0 to about -2.1 calories per gram as measured by differential scanning calorimetry;   (d) cooling the crystalline composition provided by step (c) at a rate of from between about 5° C. to about 80° C. per minute, for a time sufficient to provide a shaped article comprising said polyethylene terephthalate homopolymer, with said shaped article having an oxygen permeability of from about 0.2 to about 4.9 cc-mil/100 in 2  -24 hrs-atm at 23° C. and at 100% relative humidity inside and 60% relative humidity outside.   
     
     
       2. The process as set forth in claim 1, step (a) comprising heating said composition to a temperature greater than the crystallization onset temperature from melt of said polyethylene terephthalate homopolymer. 
     
     
       3. The process as set forth in claim 2, step (a) comprising heating said composition to from between about 500° F. to about 580° F. 
     
     
       4. The process as set forth in claim 1, wherein in step (b) said surface is at the peak crystallization rate temperature of said polyethylene terephthalate homopolymer. 
     
     
       5. The process as set forth in claim 1, wherein in step (b) said surface is the shaping surface of a mold. 
     
     
       6. The process as set forth in claim 1, wherein in step (c) contact is maintained between said surface and said polyester for from about 20 seconds to about 1 minute. 
     
     
       7. The process as set forth in claim 6, wherein in step (c) contact is maintained between said surface and said polyester for from about 20 seconds to about 40 seconds. 
     
     
       8. The process as set forth in claim 1, wherein in step (d) the the heat of recrystallization of said article is from about 0 to about -1.5 calories per gram as measured by differential scanning calorimetry. 
     
     
       9. The process as set forth in claim 8, wherein in step (d) the heat of recrystallization of said article is from about 0 to about -1.0 calories per gram as measured by differential scanning calorimetry. 
     
     
       10. The process as set forth in claim 1, step (d) comprising cooling said crystalline composition at a rate of between from about 5° C. per minute to about 50° C. per minute. 
     
     
       11. The process as set forth in claim 10, step (d) comprising cooling said crystalline composition at a rate of between from about 15° C. per minute to about 20° C. per minute. 
     
     
       12. The process as set forth in claim 1, wherein in step (d) said shaped article is microwavable, and ovenable up to a temperature of about 200° C. 
     
     
       13. The process as set forth in claim 12, wherein in step (d) said shaped article is ovenable and microwaveable food container. 
     
     
       14. The process as set forth in claim 13, wherein in step (d) said food container is a tray, box, pouch, packet or bottle, jar or cup. 
     
     
       15. The process as set forth in claim 1, wherein in step (d) said shaped article is a liquid container. 
     
     
       16. The process as set forth in claim 15, wherein in step (d) said liquid container is a bottle, jar or cup. 
     
     
       17. The process as set forth in claim 1, wherein in step (d) said oxygen permeability being from about about 0.2 to about 4 cc-mil/100 in 2  -24 hrs-atm at 23° C. and at 100% relative humidity inside and 60% relative humidity outside. 
     
     
       18. The process as set forth in claim 17, wherein in step (d) said oxygen permeability being from about 0.2 to about 3 cc-mil/100 in 2  -24 hrs-atm at 23° C. and at 100% relative humidity inside and 60% relative humidity outside. 
     
     
       19. The process as set forth in claim 18, wherein in step (d) said oxygen permeability being from about 0.2 to about 2 cc-mil/100 in 2  -24 hrs-atm at 23° C. and at 100% relative humidity inside and 60% relative humidity outside. 
     
     
       20. The process as set forth in claim 19, wherein in step (d) said oxygen permeability being from about 0.2 to about 1 cc-mil/100 in 2  -24 hrs-atm at 23° C. and at 100% relative humidity inside and 60% relative humidity outside. 
     
     
       21. The process as set forth in claim 1, wherein in step (d) said article having a linear dimensional shrinkage of between about 0% and about 6% when heated from about -60° C. to about 200° C. 
     
     
       22. The process as set forth in claim 21, wherein in step (d) said article having a linear dimensional shrinkage of between about 0% and about 3% when heated from about -60° C. to about 200° C. 
     
     
       23. A process for producing a shaped article comprising the steps: (a) heating a substantially non-oriented thermoplastic, crystallizable polyethylene terephthalate homopolymer to a completely or substantially completely amorphous state;   (b) maintaining the polyethylene terephthalate heated in step (a) at a temperature above the peak crystallization rate temperature of said polyethylene terephthalate, while simultaneously contacting said polyethylene terephthalate with a surface which imparts shape to said composition,   said surface being at or above the peak crystallization rate temperature for said polyethylene terephthalate;   (c) maintaining the contact between said polyethylene terephthalate and said surface for from about 20 seconds to about 5 minutes to provide a crystalline polyethylene terephthalate having an enthalpy of recrystallization of about 0 to about -2.1 calories per gram as measured by differential scanning calorimetry;   (d) cooling the crystalline polyethylene terephthalate provided by step (c) at a rate of from between about 5° C. to about 80° C. per minute, for a time sufficient to provide a shaped article comprising said polyethylene terephthalate, with said shaped article having an oxygen permeability of from about 0.2 to about 4.9 cc-mil/100 in 2  -24 hrs-atm at about 23° C. and at 100% relative humidity inside and 60% relative humidity outside.   
     
     
       24. A process for producing a shaped article comprising the steps: (a) heating a substantially non-oriented thermoplastic, crystallizable polyethylene terephthalate to a completely or substantially completely amorphous state;   (b) maintaining the polyethylene terephthalate heated in step (a) at a temperature above the peak crystallization rate temperature of said polyethylene terephthalate, while simultaneously contacting said polyethylene terephthalate with a surface which imparts shape to said competition,   said surface being at the peak crystallization rate temperature for said polyethylene terephthalate;   (c) maintaining the contact between said polyethylene terephthalate and said surface for from about 20 seconds to about 5 minutes to provide a crystalline polyethylene terephthalate having an enthalpy of recrystallization of about 0 to about -2.1 calories per gram as measured by differential scanning calorimetry;   (d) cooling the crystalline polyethylene terephthalate provided by step (c) at a rate of from between about 5° C. to about 80° C. per minute, for a time sufficient to provide a shaped article comprising said polyethylene terephthalate, with said shaped article having an oxygen permeability of from about 0.2 to about 4.9 cc-mil/100 in 2  -24 hrs-atm at about 23° C. and at 100% relative humidity inside and 60% relative humidity outside.   
     
     
       25. The process as set forth in any one of claims 1 to 22 wherein the composition also contains at least one additive selected from the group of nucleants, colorants, heat stabilizers, mold release agents and trimming agents. 
     
     
       26. The process as set forth in claim 23 wherein step (a) also includes at least one additive selected from the group of nucleants, colorants, heat stabilizers, mold release agents, and trimming agents. 
     
     
       27. The process as set forth in claim 24 wherein step (a) also includes at least one additive selected from the group of nucleants, colorants, heat stabilizers, mold release agents and trimming agents.